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Endocrine Reviews 25(3):374 –388
Copyright © 2004 by The Endocrine Society
doi: 10.1210/er.2003-0016
Testosterone Effects on the Breast: Implications for
Testosterone Therapy for Women
WORALUK SOMBOONPORN
AND
SUSAN R. DAVIS
The National Health and Medical Research Council Centre of Clinical Research Excellence for the Study of Women’s Health
at the Jean Hailes Foundation and the Department of Obstetrics and Gynaecology, Monash University, Clayton, Victoria
3168, Australia
Androgens have important physiological effects in women.
Postmenopausal androgen replacement, most commonly as
testosterone therapy, is becoming increasingly widespread.
This is despite the lack of clear guidelines regarding the
diagnosis of androgen insufficiency, optimal therapeutic
doses, and long-term safety data. With respect to the breast
specifically, there is the potential for exogenous testosterone to exert either androgenic or indirect estrogenic actions, with the latter potentially increasing breast cancer
risk. In experimental studies, androgens exhibit growthinhibitory and apoptotic effects in some, but not all, breast
cancer cell lines. Differing effects between cell lines appear
to be due primarily to variations in concentrations of specific coregulatory proteins at the receptor level. In rodent
breast cancer models, androgen action is antiproliferative
and proapoptotic, and is mediated via the androgen recep-
tor, despite the potential for testosterone and dehydroepiandrosterone to be aromatized to estrogen. The results from
studies in rhesus monkeys suggest that testosterone may
serve as a natural endogenous protector of the breast and
limit mitogenic and cancer-promoting effects of estrogen on
mammary epithelium. Epidemiological studies have significant methodological limitations and provide inconclusive
results. The strongest data for exogenous testosterone therapy comes from primate studies. Based on such simulations,
inclusion of testosterone in postmenopausal estrogenprogestin regimens has the potential to ameliorate the stimulating effects of combined estrogen-progestin on the breast.
Research addressing this is warranted; however, the number
of women that would be required for an adequately powered
randomized controlled trial renders such a study unlikely.
(Endocrine Reviews 25: 374 –388, 2004)
I. Introduction
I. Introduction
A
II. Testosterone Production and Metabolism
A. Biosynthesis of testosterone
B. Factors affecting circulating testosterone levels
III. Mammary Epithelial Cell Proliferation and Apoptosis
NDROGENS HAVE IMPORTANT physiological effects in women. Not only are they the precursor hormones for estrogen biosynthesis in the ovaries and extragonadal tissues (1), but androgens act directly via androgen
receptors (ARs) throughout the body. Androgen levels decline with increasing age in women before menopause (2, 3),
and it is now accepted that many postmenopausal women
experience a variety of physical symptoms secondary to androgen depletion, as well as physiological changes that affect
their quality of life (4). Postmenopausal androgen replacement, most commonly as testosterone therapy, is becoming
increasingly widespread. This is despite the lack of clear
guidelines regarding the diagnosis of androgen insufficiency, optimal therapeutic doses, and long-term safety data.
With respect to the breast specifically, there is the potential
for exogenous testosterone to exert either androgenic or indirect estrogenic actions, with the latter potentially increasing breast cancer risk.
There is justifiable concern that combined oral estrogen
plus progestin therapy significantly increases the risk of
breast cancer in postmenopausal women (5–11). Although
the underlying mechanism by which the development of
breast cancer is increased in women taking combined hormone therapy is not understood, there is a considerable
amount of evidence that androgens protect against estrogen’s mitogenic and cancer-promoting effects on breast tissue. Labrie et al. (3) have recently reviewed the role of adrenal
androgens in breast cancer growth with specific attention to
dehydroepiandrosterone (DHEA). With the increasing in-
IV. Preclinical Studies of the Effects of Androgens in Breast
Tissue
A. Breast cancer cell line studies
B. Animal studies
V. Studies in Humans of the Effects of Androgens on Breast
Cancer
A. Endogenous circulating testosterone and breast
cancer
B. Exogenous testosterone therapy and breast cancer risk
C. Implications of the detection of the AR in human
breast cancer
VI. Aromatization and Breast Cancer Development
VII. Should Androgens Be Included in Postmenopausal Hormone Therapy Regimens?
VIII. Conclusion
Abbreviations: A, Androstenedione; AR, androgen receptor; CI, confidence interval; DHEA, dehydroepiandrosterone; DHEA-S, DHEA sulfate; DHT, dihydrotestosterone; DMBA, dimethylbenz(a)antracene; ER,
estrogen receptor; OR, odds ratio; PCOS, polycystic ovarian syndrome;
PR, progesterone receptor; PSA, prostate-specific antigen; RR, relative
risk.
Endocrine Reviews is published bimonthly by The Endocrine Society
(http://www.endo-society.org), the foremost professional society serving the endocrine community.
374
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Endocrine Reviews, June 2004, 25(3):374 –388
375
clusion of testosterone in hormonal regimens, the modulating effects of this steroid on the development of breast cancer
therefore require consideration.
Thus, we have reviewed the published literature specifically pertaining to clinical studies of endogenous testosterone and testosterone therapy and breast cancer risk in premenopausal and postmenopausal women and examined the
potential benefit or risk with regard to breast cancer of the
administration of testosterone as part of hormone therapy.
II. Testosterone Production and Metabolism
A. Biosynthesis of testosterone
The term “androgens” refers to a group of 19-carbon steroid hormones that are associated with maleness and the
induction of male secondary sexual characteristics. In
women, androgens circulate in the concentration range
nanomolar to micromolar, in contrast to the estrogens, the
circulating concentrations of which are in the picomolar
range. The major androgens in women include testosterone
and dihydrotestosterone (DHT) because both have high
binding affinity to the AR. Biosynthesis of the androgens
takes place both in the adrenal and in the ovary and is
modulated by two cytochrome P450 enzymes, P450 Scc,
which catalyzes cholesterol side-chain cleavage, and P450
C17, which catalyzes 17-hydroxylation and 17–20 bond cleavage (17/20 lyase), which is required for the production of
DHEA and androstenedione (A) from pregnenolone and
progesterone, respectively. The further metabolism of androgens involves other important enzymes including 3␤hydroxysteroid dehydrogenase, catalyzing the conversion of
pregnenolone to progesterone and DHEA to A, and 17␤hydroxysteroid dehydrogenase, which catalyzes the conversion of A to testosterone. DHEA secretion is acutely stimulated by ACTH (12, 13); however, DHEA sulfate (DHEA-S),
which has a long plasma half-life, does not increase acutely
after ACTH administration (14).
DHEA-S, DHEA, and A are considered to be proandrogens because they require conversion to testosterone to exhibit androgenic effects. Up to 25% of testosterone may be
derived from the adrenal glands, 25% is derived from the
ovary, and the remaining 50% is derived from peripheral
conversion of the proandrogens, with A being the main precursor (15) (Fig. 1A). Circulating testosterone can be converted to DHT by 5␣-reductase, type 1 and type 2, or to
estradiol by the aromatase enzyme. These conversions occur
primarily in target tissues. DHT is a nonaromatizable androgen (16, 17) (Fig. 1B). Thus, androgens may exert biological effects by acting directly via the AR or indirectly after
conversion to estrogen (17, 18).
B. Factors affecting circulating testosterone levels
Under normal physiological conditions, only 1–2% of total
testosterone circulates free, unbound to plasma protein. The
rest is bound by SHBG and albumin, with SHBG binding 66%
of total circulating testosterone (19). The binding affinity for
steroids bound by SHBG is DHT ⬎ testosterone ⬎ androstenediol ⬎ estradiol ⬎ estrone (20). SHBG also weakly binds
FIG. 1. Androgen production and metabolism. A, The adrenal glands
produce all proandrogens, whereas the ovaries produce only DHEA,
A, and testosterone. In postmenopausal women, only about 25% of
circulating testosterone is directly secreted by the ovaries. The rest,
50 –75%, is formed largely from circulating precursors. B, Testosterone can be converted to DHT by 5␣-reductase type 1 and type 2 or to
estradiol by the aromatase enzyme.
DHEA, but not DHEA-S (20). SHBG is a pivotal determinant
of the bioavailability of sex steroids, and variations in the
plasma levels of SHBG impact significantly on the amount of
free and other bound sex steroids (20). Elevations in estradiol
(as occurs during pregnancy, hyperthyroidism, and liver
disease) cause a marked increase in SHBG levels, whereas
hypothyroidism, obesity, and hyperinsulinemia are associated with decreased SHBG levels (21). Standard-dose oral
estrogen, as used in hormone therapy, will increase SHBG
with little or no effect seen with standard estradiol patch
therapy (22).
In premenopausal women with regular menstrual cycles,
there is a rise in testosterone and A in the late follicular phase
of the menstrual cycle and in the luteal phase (23, 24). There
is also a diurnal variation in testosterone in women with the
peak in the morning (25). Zumoff et al. (2) showed lower
mean 24-h values for total and calculated free testosterone
among older vs. younger reproductive aged women (n ⫽ 33
women). Most recently, a study of 149 healthy premenopausal women with regular cycles, no exogenous hormone
therapy, and no complaint of low libido showed a statistically significant decline with age for free testosterone,
DHEA-S, A, and DHT, each measured after organic solvent
extraction by validated methodology (26). In the late reproductive years there is failure of the midcycle rise in free
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Endocrine Reviews, June 2004, 25(3):374 –388
Somboonporn and Davis • Testosterone Effects on the Breast
testosterone that characterizes the menstrual cycle in young
ovulating women (27). This occurs despite preservation of
normal free testosterone levels at other phases of the cycle.
The mean plasma concentrations of testosterone in women
transiting the menopause are also significantly lower than
those of younger ovulating women sampled in the early
follicular phase (28). Across the perimenopausal period, neither A, DHT, or the ratio of total testosterone to SHBG (the
free androgen index) appear to change acutely (19, 28). Controversy remains as to whether the postmenopausal ovary is
a significant source of androgen production. Concentrations
of testosterone in the ovarian vein of postmenopausal
women have been shown to be higher than those in systemic
venous blood, suggesting that the postmenopausal ovary
continues to be an androgen-secreting organ (24). In addition, testosterone levels decrease in postmenopausal women
after oophorectomy (29). However, Couzinet et al. (30) have
proposed that the postmenopausal ovary is not a major
source of androgens. Some postmenopausal women have
elevated ovarian androgen production, i.e., hyperthecosis, a
well-established but poorly studied entity. It may well be that
the androgen production of the postmenopausal ovary is
variable. This variability requires further study along with
associated factors.
of the menstrual cycle than in the follicular phase (34, 35). In
the luteal phase, both estrogen and progesterone levels are
maximal (36 –38). Free testosterone and A levels peak during
the middle-to-late follicular phase of the menstrual cycle and
remain moderately elevated up through the midluteal phase
(39).
In vitro studies of normal breast tissue are important to
understand physiological regulation of the mammary gland
by sex steroids. However, this has been hampered by experimental difficulties (40). One problem has been that normal human breast cells lose their steroid receptors and,
hence, their hormone responsiveness almost as soon as they
are isolated and placed into culture (40). Only one study of
hormone-responsive primary cultures of breast epithelial
cells reported that estradiol, but not progesterone, stimulated
proliferation (41).
In vitro studies have consistently demonstrated that estradiol is a major mitogen in breast cancer cell lines (42– 44). In
breast cancer cell lines, cell death has been reported to be
induced by estrogen deprivation (45) or antiestrogen (46). In
contrast, in vivo evidence that supports a role for progesterone in cell proliferation in the breast has been difficult to
reproduce in vitro (47).
IV. Preclincial Studies of the Effects of Androgens in
Breast Tissue
III. Mammary Epithelial Cell Proliferation and
Apoptosis
Steroids and their nuclear receptors play crucial roles in
the development and maintenance of normal functions of the
human mammary gland. In addition to estrogen receptor-␣
(ER␣), estrogen receptor-␤ (ER␤), and progesterone receptors (PRs), ARs are present in both normal mammary tissue
and breast cancer cell lines (31, 32). Hormone stimulation of
mammary epithelial proliferation and apoptosis is important
in tissue homeostasis. Deregulation of apoptosis can promote
tumorogenesis as well as proliferation (33).
In premenopausal women, proliferation and apoptosis of
normal breast epithelial tissue are higher in the luteal phase
A. Breast cancer cell line studies
There is no in vitro evidence pertaining to the effects of
androgens on normal human breast cells. Studies of the effects of androgens in various breast cancer cell lines predominantly support apoptotic and antiproliferative effects of
androgens on the mitogenic effects of estrogens. However,
divergent findings have been reported with differences according to the specific cell line, the androgen used, and its
dose and estrogen status. The effects of testosterone and DHT
in breast cancer cell lines are summarized in Table 1.
1. Antiproliferative effects. The antiproliferative effects depend
on the following factors.
TABLE 1. Effects of testosterone and DHT on breast cancer cell lines
Study
Model
Type of
androgen
Dose of androgen
(M )
⫺11
Main outcome
measurement
⫺7
Proliferation
Boccuzzi et al., 1994 (51)
MCF-7
DHT
20
Birrell et al., 1995 (52)
MCF-7, ZR75-1, TD47-D,
MDA-MB-453
DHT
10⫺10–10⫺8
Proliferation
Ando et al., 2002 (48)
MCF-7
Ortmann et al., 2002
(54)
MCF-7, T47-D, BT-20,
MDA-MB 435S
Testosterone
DHT
Testosterone
10⫺9–10⫺6
10⫺9 –10⫺6
10⫺9–10⫺7
Proliferation
Proliferation
Proliferation
DHT
10⫺9–10⫺7
Proliferation
DHT
DHT
10⫺8
10⫺9
Apoptosis
Bcl-2
protooncogene
Kandouz et al., 1999 (58)
Lapointe et al., 1999 (61)
MCF-7, ZR75-1, T47-D
ZR-75-1
–20
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Result
Biphasic effect: stimulation
at a very high
concentration; inhibition
at concentration up to
20⫺8 M
Stimulation in the MCF-7
and MDA-MB-453; no
effect in MDA-MB-231 or
BT-20, inhibition in T47-D
and ZR-75-1
Inhibition
Inhibition
Inhibition in all four cell
lines
Inhibition in all four cell
lines
Proapoptotic effect
Down-regulation
Somboonporn and Davis • Testosterone Effects on the Breast
a. Estrogen status and type of androgen used. Ando et al. (48)
simulated the hormonal environment in pre- and postmenopausal women with an in vitro model utilizing the ER-positive breast cancer cell line MCF-7 exposed to DHEA,
DHEA-S, androstenediol, testosterone, and DHT with or
without estradiol. They found that DHEA-S and androstenediol stimulated the growth of MCF-7 cells in the absence
of estradiol, but reduced cell proliferation in the presence of
estradiol at 1 nmol/liter. This is consistent with the possibility of DHEA-S being converted to estrone and hence to
estradiol, and androstenediol acting as a weak estrogen (49).
Testosterone and DHT, at 1–100 nmol/liter, inhibited MCF-7
cell proliferation independently of the presence of estradiol.
DHT alone, at 100 nmol/liter, consistently inhibited MCF-7
cell proliferation by 50% of the basal growth rate and counteracted estradiol-proliferative action by 68%. Normal circulating levels in women are approximately 0.5–2.3 nmol/
liter for testosterone, and 0.2– 0.8 nmol/liter for DHT. Thus,
most of the concentrations for these hormones used in this
and other studies are supraphysiological. Cell cycle analysis
showed an enhanced number of cells in G0/G1 phase after 6 d
of DHT treatment. Moreover, upon prolonged DHT exposure, a markedly increased AR content, mostly translocated
into the nucleus, was observed. The inhibitory effect of DHT
on cell proliferation was lost when the cells were treated with
the AR antagonist, hydroxyflutamide (48). In cotransfection
experiments, overexpression of the AR decreased estradiolinduced signaling (48). This was amplified by treatment with
DHT but lost with the addition of hydroxyflutamide. When
the cells were cotransfected with a mutant AR, inhibition of
estradiol-induced signaling did not occur. Thus, direct androgen action appears to antagonize MCF-7 proliferation
induced by estradiol, and this seems to be related to the
inhibitory effects of the AR on estradiol genomic action (48).
These experimental results are consistent with those of Birrell
et al. (50) who proposed that the therapeutic action of medroxyprogesterone acetate in breast cancer may be partially
mediated by the AR.
b. Androgen concentration. The effects of DHT appear to be
concentration dependent. Boccuzzi et al. (51) reported that, at
an extremely high concentration (200 nmol/liter), DHT stimulated MCF-7 cell growth through an ER-mediated mechanism, whereas lower concentrations of DHT were inhibitory.
c. Type of breast cancer cell line. The effects on proliferation
in vitro vary according to the androgen administered and the
breast cancer cell line studied (52, 53). At concentrations of
1 nmol/liter for 10 d, which is close to the normal female
physiological range, Birrell et al. (52) reported varying stimulatory and inhibitory effects of DHT on differing breast
cancer cell lines that simply could not be explained by varying hormone receptor status. The two cell lines that had no
hormone receptors did not respond to treatment. DHT stimulated both the ER-positive MCF-7 cells and the ER-negative
MDA-MB-453 cells. Treatment with 100-fold excess of hydroxyflutamide reversed the effects of DHT in each of the cell
lines. In the same study (data not shown) the synthetic nonmetabolizable androgen mibolerone had effects similar to
those of DHT, with the exception that hydroxyflutamide only
Endocrine Reviews, June 2004, 25(3):374 –388
377
partially reversed the growth-stimulatory effects of this treatment on MCF-7 and MDA-MB-453 cells. Hydroxyflutamide
only partially reversed the inhibitory effects of DHT on ZR75–1 cultures, whereas AR antisense oligonucleotides reversed the growth-inhibitory action of miberolone in this cell
line.
These observations support the theory that AR expression
is a necessary requirement for androgenic effects on breast
cancer cell proliferation, but that the absolute levels of AR (as
well as ER and PR) in cell lines are associated with neither
a specific stimulatory nor inhibitory proliferative responses
(52). It is likely, therefore, that additional cellular factors or
the structure of the AR determine whether breast cancer cell
proliferation is stimulated or inhibited in the presence of
androgen (53).
In contrast to the above, Ortmann et al. (54) reported dosedependent inhibition with androgens of four cell lines that
each stained positively for the AR. Included among these was
the BT-20 cell line, which was reported by two other groups
to be AR negative (52, 55). According to proliferation kinetics,
they observed a significant (P ⬍ 0.05) dose-dependent inhibition of cell growth by both testosterone and DHT. In the
ER-negative cell lines, BT-20 and MDA-MB 435S testosterone
was a more potent inhibitor of cell proliferation than DHT
(P ⬍ 0.05), whereas in the ER-positive cells lines, MCF-7 and
T47-D, stronger inhibition of proliferation was achieved with
DHT than with testosterone. They proposed that partial
transformation of testosterone to estrogen in ER-positive
cells might be an explanation for this effect (54).
Prostate-specific antigen (PSA) is a new favorable prognostic indicator for women with breast cancer (56). Immunoreactive PSA has been reported in 30% of breast cancers
and has been associated with both earlier stage disease and
longer relapse-free survival (56). KLK3 (which encodes PSA)
and KLK2 (encoding human kallikrein 2) are both known to
be androgen regulated, but respond differentially to androgens when studied in different human breast cancer cell lines
(57). Research into the various factors affecting the production of these two proteins, according to the breast cancer cell
line studied, gives insight into how androgen treatment may
affect different cell lines differently.
Initial experiments demonstrated that the differential androgen induction of PSA and human kallikrein 2 was not
directly related to the level of AR expression or to mutations
within the coding region (55). Because the action of a steroid
receptor at a given promoter may be modulated by various
coregulatory proteins (coactivators/corepressors), Magklara
et al. (55) examined the expression of various known coregulatory proteins in the different breast cancer cell lines (BT474, T-47D, ZR75–1, MCF-7, MFM-223 and BT-20). They
found that the mRNA levels of steroid receptor coactivator
1, a known coactivator of the activation function 1 domain of
the AR, were highest in the breast cancer cell lines with the
greatest PSA production and lowest in the cell lines that
secreted less PSA. This raises the possibility that the relative
levels of specific coactivators/corepressors might differentially modulate AR transcriptional activity within the promoter/enhancer region of KLK2 and KLK3 of different
breast cancer cell lines (55).
Thus, the ultimate effects of testosterone and DHT at the
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Endocrine Reviews, June 2004, 25(3):374 –388
tissue level may not be influenced simply by absolute ligand
concentrations, but by the relative concentrations of specific
coregulatory proteins unique to each cell line. Hence, the
effects of DHT differ between different breast cancer cell
lines, despite the consistent presence of ARs. In the ARpositive human breast cancer cell lines, T47-D and ZR-75–1,
DHT has been reported to be proapoptotic, with maximal
effects at 10 nmol/liter (58). These findings are in line with
the growth-inhibitory effects reported by Birrell et al. (50) in
these two cell lines.
2. Apoptotic effects. The effects of androgens on the expression of two genes known to influence apoptosis, Bcl-2 and
Bax, have been investigated. Bcl-2 is able to counteract
apoptosis induced by numerous stimuli such as UV light,
␥-radiation, heat shock, and chemotherapy and thus is
considered antiapoptotic (59), and Bax is a proapoptotic
gene (60). Lapointe et al. (61) reported that DHT downregulated Bcl-2 protooncogene levels via an AR-mediated
mechanism in the ZR75–1 breast cancer cell line in either
the presence or absence of 17␤-estradiol. This is consistent
with the inhibitory effect of DHT in this cell line. Inhibition
by DHT was completely prevented by coincubation with
the pure antiandrogen hydroxyflutamide (61). Xie et al.
(60) studied Bcl-2 and Bax expression in the Noble rat
model, which they had established to explore the mechanisms of hormonal mammary carcinogenesis. They observed that Bcl-2 was strongly expressed in most of the
mammary tumor cells, and that when animals were
treated with 17␤-estradiol, the mammary epithelial cells
expressed higher levels of Bcl-2. Bax immunoreactivity
was weak in mammary tumor cells but strongly expressed
in adjacent normal or hyperplastic ductal structures.
Treatment with testosterone, either alone or in combination with estrogen, gave rise to high levels of Bax expression in “premalignant” mammary glands. This supports
the hypothesis that testosterone may oppose the mitogenic
action of estrogen in the breast by promoting apoptosis
(62).
Taken together, androgens exhibit growth-inhibitory and
apoptotic effects in some, but not all, breast cancer cell lines.
Differing effects between cell lines appear to be primarily due
to variations in concentrations of specific coregulatory proteins at the receptor level. In rodent breast cancer models,
androgen action is antiproliferative and proapoptotic, and
mediated via the AR, despite the potential for testosterone
and DHEA to be aromatized to estrogen.
B. Animal studies
1. DHEA. The effects of DHEA have been renewed in detail
recently (3). In summary, data from in vivo studies support
a primarily inhibitory effect of testosterone on the proliferative effects of estrogen. Dimethylbenz(a)anthracene
(DMBA)-induced mammary carcinoma in the rat is a commonly used model for in vivo studies of the prevention and
treatment of breast cancer. Labrie and associates (63– 65)
and Lubet et al. (66) have used this model extensively to
study the effects of DHEA and have consistently reported
an inhibitory effect of DHEA on mammary carcinoma de-
Somboonporn and Davis • Testosterone Effects on the Breast
velopment. Treatment with SILASTIC (Dow Corning Corp.,
Midland, MI) implants of DHEA leading to serum DHEA
levels comparable to those observed in normal adult women
(7.1 ⫹ 0.6 nmol/liter and 17.5 ⫹ 1.1 nmol/liter) progressively inhibited the development of DMBA-induced mammary carcinoma in the rat (63). Luo et al. (64) also demonstrated inhibition in DMBA-induced mammary carcinoma
development: 279 d after DMBA administration, the incidence of mammary carcinoma had decreased from 95% in
control animals to 73% (P ⬍ 0.05), 57% (P ⬍ 0.01), and 38%
(P ⬍ 0.01) at the daily percutaneous doses of 5, 10, and 20
mg of DHEA, respectively. Moreover, the mean tumor number and the mean tumor area per tumor-bearing animal
were also reduced by the same treatments. Similar outcomes
have been reported in a N-methyl-N-nitrosourea-induced
rat mammary tumor model with DHEA therapy (65), and a
suppressive effect of DHEA has been demonstrated on the
growth of estrone-stimulated sc tumor xenografts formed
by AR-positive ZR-75–1 cells in ovariectomized nude mice
(66).
2. Testosterone and DHT. Table 2 summarizes the effects of
testosterone and DHT in animal studies. There is both direct
and indirect evidence of the inhibitory effects of testosterone
and DHT on DMBA-induced mammary carcinoma in ovariectomized rats (67, 68). DHT therapy reduced the number of
progressing tumors from 69.2 to 29.2% in estradiol-treated
animals, and the number of tumors that completely regressed increased from 11.5 to 33.3% in the same groups of
animals (67). Simultaneous treatment with the antiandrogen
flutamide completely prevented the inhibitory effect of DHT
on tumor growth (67). DHT exhibited similar tumor-suppressing effects on ZR-75-1 breast cancer cells implanted into
ovariectomized athymic mice in either the presence or absence of exogenous estradiol (69).
Zhou et al. (31) explored the effects of estrogen, progesterone, and testosterone on normal mammary epithelial cell
proliferation and steroid receptor gene expression in the
ovariectomized primate mammal. They showed that estrogen therapy alone significantly increased mammary epithelial proliferation approximately 6-fold and significantly increased the mammary epithelial level of ER␣ mRNA.
Progesterone administration did not modify the proliferative
effects of estradiol significantly. When given concurrently,
testosterone reduced estradiol-induced epithelial proliferation by approximately 40% and entirely abolished the estradiol-induced augmentation of ER␣ gene expression. However, testosterone levels achieved in this study were
supraphysiological. Zhou et al. (31) also investigated the effects of tamoxifen and found that it caused a 3-fold increase
in mammary epithelial proliferation, measured by Ki67, but
a decrease in ER␣ gene expression below placebo level. AR
mRNA levels detected by in situ hybridization were not
altered by estradiol alone, but were significantly reduced by
estradiol plus testosterone or tamoxifen treatment. Thus, testosterone induced down-regulation of mammary epithelial
proliferation and ER␣ gene expression. This suggests that the
addition of testosterone might reduce the risk of breast cancer associated with estrogen-progestin therapy in postmenopausal women. In addition to the parallel effects of tamoxifen
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Endocrine Reviews, June 2004, 25(3):374 –388
379
TABLE 2. Effects of testosterone and DHT in animal studies
Study
Dauvois et al., 1989
(67)
Dauvois et al., 1991
(69)
Jayo et al., 2000
(71)
Zhou et al., 2000
(31)
Dimitrakakis et al.,
2003 (70)
Model
Main outcome
measurement
Result
The number of
progressing tumors
The number of
complete responses
The number of
progressing tumors
The number of complete
responses
Significant decrease with E ⫹ DHT
vs. E alone
Significant increase with E ⫹ DHT
vs. E alone
Significant decrease with E ⫹ DHT
vs. E alone
Significant decrease with E ⫹ DHT
vs. E alone
Oral contraceptive
vs. oral
contraceptive ⫹
methyltestosterone
E; E ⫹ progesterone;
E⫹T
Mammary epithelial
proliferation
Significant decrease with oral
contraceptive ⫹ testosterone vs.
oral contraceptive alone
Mammary epithelial
proliferation
Significant decrease with E ⫹ T vs.
others
Flutamide vs. none
Mammary epithelial
proliferation
Significant increase in flutamide
E; E ⫹ progesterone;
E ⫹ T; vehicle
Mammary epithelial
proliferation
ER␣/ER␤
MYC expression
Significant decrease with E ⫹ T vs.
E or E ⫹ progesterone
Reversal of ER␣/ER␤
Significant reduction
Intervention
DMBA-induced
mammary
carcinoma in
rats
ZR-75-1 breast
cancer cells
implanted
into
ovariectomized
athymic mice
Rat
Ovariectomized
rhesus
monkeys
Normal-cycling
rhesus
monkeys
Ovariectomized
rhesus
monkeys
E vs. E ⫹ DHT
E vs. E ⫹ DHT
E, Estradiol; T, testosterone.
on primate mammary epithelial sex steroid receptor gene
expression, Zhou et al. (31) also demonstrated that tamoxifen,
like testosterone, reduced apolipoprotein D mRNA levels
and increased IGF binding protein 5 expression in the primate mammary gland. These findings support AR-mediated
effects of tamoxifen.
Subsequently, Dimitrakakis et al. (70) have investigated
the effects of ovarian steroids in physiological doses on the
mammary epithelial proliferation in ovariectomized rhesus
monkeys. They studied four groups treated with placebo,
estradiol alone, estradiol plus progesterone, or estradiol plus
testosterone. Circulating estradiol levels were similar in the
three active treatment groups, and testosterone levels were
physiological. The mammary epithelial proliferation index
was measured using Ki67 immunoreactivity. Estradiol alone
and estradiol plus progesterone resulted in a significantly
increased mammary epithelial proliferation index compared
with placebo controls by approximately 3.5-fold, whereas the
estradiol plus testosterone combination did not increase the
proliferation index above control values (70). In addition,
they found a significant reduction in mammary epithelial
ER␣ and increase in ER␤ expression in estradiol plus testosterone groups compared with estradiol alone. This effect
of testosterone resulted in a reversal of the ER␣/ER␤ ratio,
which was approximately 2.5 in the estradiol-treated group
and approximately 0.7 in the estradiol-testosterone group.
Moreover, testosterone treatment was associated with an
approximate 50% reduction in mammary epithelial MYC
expression, an estrogen-responsive gene, compared with the
estradiol- and estradiol-progesterone-treated groups (P ⫽
0.05), suggesting that the antiestrogenic effects of testosterone at the mammary gland involve alteration in ER signaling
to MYC (70). They further investigated the importance of
endogenous testosterone in intact, cycling monkeys by
studying treatment with either placebo or the AR antagonist
flutamide. The mammary epithelial proliferation index increased 2-fold after treatment with flutamide alone (P ⫽
0.03). This suggests that testosterone may serve as a natural
endogenous protector of the breast and may limit mitogenic
and cancer-promoting effects of estrogen on the mammary
epithelium (70). Jayo et al. (71) also reported that oral contraceptive therapy plus methyltestosterone in rats causes
significant suppression of epithelial cell proliferation, a reduction in the number of proliferating cell nuclear antigenlabeled cells, and an increase in the number of PR-labeled
cells. However, there have been no studies of the effects of
testosterone plus estrogen-progestin therapy on breast epithelial cell proliferation in women.
In ovariectomized 12-month-old rats, DHT stimulated
mammary gland lobulo-alveolar ductal growth (72). Medroxyprogesterone acetate induced the same effect, consistent with the findings of Birrell et al. (50) that androgens and
medroxyprogesterone acetate share a similar mechanism of
action in the breast. DHEA also stimulated lobulo-alveolar
development, which was unaffected by the antiestrogen EM800 but almost completely prevented by cotreatment with the
antiandrogen flutamide (47, 72).
Taken together, available in vivo data indicate that
in estrogen-replete normal breast tissue, androgens diminish estrogen-induced breast epithelial proliferation and
abolish estrogen-induced ER␣ gene expression in the primate. In the absence of estrogen, androgen action mimics
that of progestins in the rat mammary gland.
Thus results from the studies in vitro and in vivo suggest
that testosterone may serve as a natural endogenous protector of the breast and limit mitogenic and cancer-promoting effects of estrogen on mammary epithelium. However,
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380
Endocrine Reviews, June 2004, 25(3):374 –388
these are surrogate endpoints and hence the need to consider
what is known from studies in women.
V. Studies in Humans of the Effects of Androgens on
Breast Cancer
A. Endogenous circulating testosterone and breast cancer
1. Issues in clinical trials. Findings from case-controlled studies of the relationship between endogenous testosterone levels and breast cancer risk do not necessarily translate to
women treated with exogenous testosterone. The former address endogenous androgen production, which in some
women may be pathophysiological. In contrast, postmenopausal testosterone therapy is administered to women who
have low testosterone levels due to low production and who
are usually also treated with exogenous estrogen. Furthermore, if an association is found between endogenous circulating testosterone and breast cancer, it does not necessarily
signify a causal relationship. Total testosterone, although the
most common measure for clinical studies to date, does not
yield specifically meaningful information about actual tissue
androgen exposure. It is widely accepted that free testosterone is the strongest indicator of tissue androgen exposure
and that variations in SHBG levels in women can have dramatic effects on free testosterone levels (4, 73, 74). In addition,
a single value may be inadequate to assess true tissue exposure because testosterone levels vary in response to diurnal rhythms (25). Stress is also an important confounder in
cross-sectional studies, because stress itself affects testosterone levels (75–78). Whether even free testosterone is a meaningful indicator of tissue androgen exposure remains controversial. Labrie et al. (79, 80) proposed that the major
proportion of androgenic effects in women are derived from
an intracrine mode of action, which will not be detected by
measurement of circulating testosterone or DHT. Regarding
the type of study, a clear inference of effect cannot be drawn
from cross-sectional data because such research cannot provide an appropriate time sequence of exposure and outcome.
Consideration also needs to be given not only to what has
been measured but also the effects of storage and the sensitivity of the assay methodology used. For example, with
long-term cryopreservation, testosterone has been shown to
increase by 5% per year of blood storage at ⫺20 C (81). This
may not apply to storage at much colder temperatures (82).
No rapid, simple assay of total testosterone has been
shown to produce reliable results in women with low to
normal testosterone levels. Direct testosterone immunoassays are limited by “noise” from assay interference and by
cross-reactivity with other steroids, which become worse at
low testosterone concentrations (83). Inclusion of an organic
solvent extraction step when measuring total testosterone
will increase the assay specificity, and if combined with
chromatographic separation of testosterone from interfering
steroids, a reliable result can be obtained. The gold standard
methodology for measurement of free testosterone is considered to be equilibrium dialysis. Measurement of free testosterone by analog assay is notoriously unreliable, particularly at the lower end of the normal female range and is not
recommended for use (83).
Somboonporn and Davis • Testosterone Effects on the Breast
Finally, because estrogen is considered a strong risk factor
for breast cancer, to draw any conclusion about an association between testosterone and breast cancer, a statistical
method to adjust for the estrogen effect must be employed.
2. Testosterone levels and breast cancer in premenopausal women.
Two cross-sectional studies have investigated the relationship between total testosterone and breast cancer risk in
premenopausal women and have yielded inconsistent results (Table 3). Secreto et al. (84) reported an age-adjusted
relative risk (RR) for high vs. low levels of serum total testosterone of 3.4 [95% confidence interval (CI), 1.6 –7.3] and for
urinary testosterone of 2.1 (95% CI, 0.9 – 4.8) for cases (n ⫽ 63)
vs. controls (n ⫽ 70). Study samples were collected between
cycle d 18 and d 21 irrespective of cycle length or whether
ovulation had or had not occurred. The association was observed only in women whose samples were collected 5–9 d
before the next menses (a period corresponding to the midluteal phase) and 10 or more days before the next menses.
There was no positive association for women whose blood
and urine were collected within 4 d of the next menses and
who thus had cycles lasting 25 d or less. The authors’ interpretation of these findings was that higher testosterone levels
were detectable in cases only in the follicular or early luteal
phases; however, these phases are the times of highest testosterone levels during the normal cycle (39). They also reported that high testosterone was characteristic of breast
cancer patients with long menstrual cycles (⬎28 d) and negligible in women with short ones (⬍28 d) but that low SHBG
levels appeared to be a protective factor for breast cancer.
Thus, the high total testosterone measured corresponded to
periovulatory hormone production but was not abnormally
high across the cycle in premenopausal women with breast
cancer, consistent with the fact that SHBG was not abnormally low in patients with breast cancer.
The most recent cross-sectional study involved 171 premenopausal women with breast cancer and 170 controls
matched by age (85). No significant difference by odds ratio
(OR) for breast cancer between high and low testosterone
was found when data were adjusted for confounding factors.
Overall, no conclusions can be made about testosterone and
breast cancer in premenopausal women based on these two
studies.
Two prospective case-control studies in which total testosterone was measured provide more consistent results (Table 3). Wysowski et al. (86) found no statistical associations
between serum hormone levels, including total testosterone,
in 17 women diagnosed with breast cancer 8 –132 months
after blood was drawn, each matched to four controls. Similarly, in a study of 62 premenopausal women with breast
cancer and 182 controls, Thomas et al. (87) found no statistical
difference for total testosterone between the groups
3. Polycystic ovarian syndrome (PCOS) and incidence of breast
cancer. In premenopausal women, PCOS is characterized by
infertility, hyperandrogenism, and obesity. Concentrations
of testosterone, A, and DHEA-S, and the calculated free
androgen index (total testosterone in nanomoles per liter/
SHBG in nanomoles per liter ⫻ 100) are significantly higher
in women with PCOS regardless of hirsutism (88). This syn-
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Somboonporn and Davis • Testosterone Effects on the Breast
Endocrine Reviews, June 2004, 25(3):374 –388
381
TABLE 3. Epidemiological studies of the association between plasma testosterone levels and risk of breast cancer in premenopausal
women: study size, characteristics, and summary results
Study
Comparison made
Case/control
(no. of women)
Type of T
measurement
Trial type
Study characteristics
Secreto et al., 1989 (84)
Cross-sectional
Matched to age (⫾ 6
months)
Top to bottom
quartile
75/150
Total T
2.1a
Yu et al., 2003 (85)
Cross-sectional
Matched to age
171/170
Total T
Wysowski et al., 1987 (86)
Prospective
17/68
Total T
3.4 (1.6 –7.3)b
1.9 (1.0 –3.7)c
2.01 (0.96 – 4.2)d
Not applicable
Thomas et al., 1997 (87)
Prospective
7-yr follow-up;
matched to race,
age, and time since
last menstrual
period
Matched to age, year
of blood collection,
and no. of years
postmenopausal
Top to bottom
tertile
Mean values
for cases vs.
controlse
61/179
Total T
1.2 (0.6 –2.4)f
1U increase in
the natural
log of
hormone
concentration
OR (95% CI)
T, Testosterone.
a
Age adjusted.
b
Adjusted for occupation and no. of children.
c
Unadjusted.
d
Adjusted for waist-hip ratio, age at first live birth, total caloric intake, fibroadenoma, and SHBG.
e
Mean comparison resulted in no statistically significant difference of testosterone levels between cases and controls.
f
Unadjusted OR.
TABLE 4. Epidemiological studies of the association between plasma testosterone levels and risk of breast cancer in women with PCOS:
study size, characteristics, and summary results
Study
Trial type
Coulam et al., 1983 (89)
Prospective
cohort
Gammon and Thompson, 1991 (91)
Case-control
Anderson et al., 1997 (90)
Prospective
cohort
Study characteristics
Comparison made
RR (95% CI)
Clinic-based study; 1270 subjects
diagnosed as having chronic
anovulatory syndrome were
followed during 1935–1980
Population-based study; 4730
women with breast cancer and
4688 control women aged 20 –
54 yr
Population-based study; 34,835
women at risk, age 55– 69 yr,
were followed during 1986 –
1992
Observed incidence of breast
cancer vs. expected
incidence based on
standard population
OR
1.5 (0.8 –2.6)
Incidence of breast cancer
among women with SteinLeventhal syndrome vs.
incidence among women
without this disease in the
same cohort
1.2 (0.7–2)b
1.0 (0.5–1.8)c
0.5 (0.3– 0.9)a
a
Age-adjusted OR for age.
Adjusted for age.
c
Adjusted for age, age at menarche, age at first pregnancy, parity, oral contraceptive use, hormone replacement therapy, body mass index,
waist-to-hip ratio, benign breast disease, and family history of breast carcinoma.
b
drome is a useful model of the effects of long-term exposure
to hormone imbalance. An increased risk of endometrial
cancer has been documented in women with this condition
(89). Table 4 summarizes the studies of PCOS and breast
cancer risk. Despite hyperandrogenism and long-term exposure to unopposed estrogen, the risk of breast cancer is not
increased in women with PCOS (89, 90). In fact, Gammon and
Thompson (91) have reported an age-adjusted OR for breast
cancer in women with this syndrome of 0.52 (95% CI, 0.32–
0.87). Although this risk reduction might be related to the
hyperandrogenemia of this condition, a cause and effect cannot be established.
4. Testosterone and breast cancer in postmenopausal women
a. Cross-sectional studies. In their cross-sectional studies,
Lipworth et al. (92) measured hormone levels in cases 1 wk
post surgery, and Secreto et al. (93) collected blood and urine
from women after diagnosis, but before surgery. Each group
felt their protocol minimized the influence of stress, but this
is questionable. Both studies reported only total testosterone,
not free or bioavailable testosterone. Secreto’s group (93)
reported that breast cancer patients with elevated urinary
testosterone levels at the time of diagnosis showed a dramatic decrease in the excretion levels of this hormone after
bilateral ovariectomy, and that histological examination of
the excised ovaries revealed hyperplasia of interstitial cells in
all hyperandrogenic patients. No such change was observed
in patients with normal testosterone levels at the time of
diagnosis preovariectomy (94). This implies that the hyperandrogenic postmenopausal women who developed
breast cancer in Secreto’s study had ovarian pathophysiology. Lipworth et al. (92) reported no increase for breast cancer
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382
Endocrine Reviews, June 2004, 25(3):374 –388
with higher testosterone levels after adjustment for age, residence, and hormonal factors (OR, 0.48; 95% CI, 0.12–1.90),
whereas Secreto et al. (93) reported a positive association after
adjustment for occupation and number of children (OR, 2.7;
95% CI, 1.1– 6.7). Secreto et al. (93) also measured DHT in
postmenopausal women with breast cancer vs. controls and
found no significant difference between the two groups.
b. Prospective case-control studies. Ten prospective case-control studies have been undertaken (81, 82, 86, 95–101). Study
designs, characteristics, and results of these studies are listed
in Table 5. All met the appropriate requirements for prevention of biochemical measurement bias, and case-control identification among these studies was similar. To address intraindividual variability of hormone levels with time,
Berrino et al. (95), Hankinson et al. (102), and Thomas et al. (81)
reported intraclass correlation coefficients between two
blood samples. Several groups measured only total testosterone (81, 82, 86, 97, 98, 100). Overall, three of the studies did
not demonstrate any significant associations without adjustment for estradiol (82, 86, 98). Dorgan et al. (96) reported the
RR for breast cancer for women in the highest vs. the lowest
quartile of total testosterone levels as being 6.2 (95% CI,
2.0 –19.0). However, they did not control for estradiol in their
calculations. Positive associations between breast cancer and
total testosterone that were no longer significant after adjustment for estradiol were demonstrated by three groups
(81, 97) or estrone (101). In contrast, Manjer et al. (100) reported an association between high total testosterone and
breast cancer risk after adjustment for estradiol. Only two
groups (95, 99) measured free testosterone. Cauley et al. (99)
measured free testosterone by equilibrium dialysis, considered to be the most accurate methodology, in 97 cases and 244
controls and reported no association between free testosterone and breast cancer after adjustment for estradiol. In contrast Berrino et al. (95) measured free testosterone by RIA in
25 breast cancer patients and 100 controls and reported a
significant association between higher free testosterone levels and breast cancer after adjustment for estradiol.
A reanalysis of prospective studies (103) reported that the
RRs associated with a doubling of total testosterone levels
were 1.37 (95% CI, 1.15–1.65) for the three studies incorporating a purification step in their testosterone assay (98, 99,
104) and 1.44 (95% CI, 1.21–1.72) for the four studies that used
a direct testosterone assay (81, 95–97). However, a subgroup
analysis to determine the association between breast cancer
and free testosterone levels was not undertaken. Thus, evidence from clinical studies that the free fraction of testosterone is an independent risk factor for breast cancer is
lacking.
B. Exogenous testosterone therapy and breast cancer risk
Three observational studies have addressed the use of
testosterone therapy and breast cancer risk (105–107). Unfortunately, the primary aim for two of these studies was not
testosterone supplementation and breast cancer risk; consequently, they each had only a small sample size for this
subgroup analysis. Brinton et al. (108) undertook a casecontrol study of postmenopausal estrogen use and breast
Somboonporn and Davis • Testosterone Effects on the Breast
cancer risk. A subgroup analysis in this study of 25 patients
and 29 controls showed no significant increase in risk with
oral methyltestosterone in combination with conjugated
equine estrogen (RR, 1.05; 95% CI, 0.6 –1.8) (108). In contrast,
Ewertz (109) studied the effects of the ever-use of im injections containing estradiol-testosterone (2.5 mg estradiol plus
50 mg testosterone or 5.0 mg estradiol plus 100 mg testosterone) given at a recommended interval of 3–7 wk in a
subgroup analysis. This specific therapy was used for 56 of
1694 patients and 21 of 1705 controls. An RR of 2.3 (95% CI,
1.37–3.88) was reported (109). In the same study, there was
no risk for triple combination of estrogen, progestin, and
testosterone (RR 1.26; 95% CI, 0.58 –2.74) (109). A recent retrospective analysis of 511 Australian women treated with
conventional estrogen therapy plus 50 –150 mg testosterone
implants with a mean follow-up of 5.7 ⫾ 2.5 yr, but no control
group, reported a breast cancer incidence of 240 per 100,000
women years (107). This was reported as equivalent to the
incidence in the general population determined by the state
cancer registry.
C. Implications of the detection of the AR in human
breast cancer
ARs are found in more than 50% of breast tumors (110),
and the significance of ARs in breast cancer has been extensively explored. With regard to nodal metastasis, Soreide et
al. (111) reported that when the median value of AR is taken
as cut-off (50.5 pmol/g), a lower AR content is an independent predictor of the likelihood of axillary metastases (P ⫽
0.001). AR amount, however, did not reveal any significant
prognostic information concerning relapse-free survival.
There appears to be no significant association between AR
expression and the degree of differentiation of ductal carcinoma in situ (112).
In addition to the amount of AR, correlations between the
repeat length of the CAG sequence in the AR and total risk
of breast cancer, age at diagnosis, recurrence after surgery,
and aggressive growth have been reported. CAG repeat
length is associated with a decreased ability to activate ARresponsive genes (113). More CAG repeats in the AR gene
have been associated with an earlier onset of breast cancer
among BRCA1 mutation carriers (114). However, Kadouri et
al. (115) found no significant association between the number
of CAG and GGC repeats in the AR and breast cancer risk in
either BRCA1/2 carriers or the general population if attention was restricted to Ashkenazi Jewish carriers, or only to
BRCA1 or BRCA2 carriers. One explanation for the discrepancy is sample size in that the larger number of study subjects
with the BRCA1 mutation in the former study (165 with and
139 without breast cancer) could provide a statistical difference rather than smaller study subjects with BRCA1 and
BRCA2 mutations (122 with and 66 without breast cancer).
Therefore, an effect of the AR repeat length on BRCA1 penetrance cannot be excluded. However, this remains inconclusive for BRCA2.
The relationship between the number of CAG and GGC
repeats has also been evaluated in a population-based study
conducted in 524 patients and 461 controls for their relationships to breast cancer risk (116). This study suggested a
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Excluded new cases
diagnosed within
6 months of
recruitment
All
All
All
Excluded new cases
diagnosed within
6 months of
recruitment
ZeleniuchJacquotte,
1997 (97)
Cauley, 1999
(99)
Manjer, 2003
(100)
ZeleniuchJacquotte,
2004 (101)
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173/438
297/563
e,h
97/243
155/310
851/163
61/179
5.4
3.2
2.4
2.7
7.8
71/133
67/264
31/287
39/156
No. of
cases/controls
60
60
61.6
60.5
Total T
Total T
Total T, free T
Total T
62c
62c
70.9
71.8
Total T
Total T
Total T
Total T, free T
Total T
Total T
Type of T assay
59.2
59.1
58.6
58.5
65.3
66.6
59.4
54.9
61.0
62.0
61.0c
61.0c
Mean or
median age
(yr)
Cohort
Matched 2:1
Subcohort
Matched 2:1
Matched 4:1
Matched 3:1
Matched 2:1
Matched 4:1
Full cohort
Matched 4:1
Ratio of control
subjects to case
patients
e
2.2 (1.3–3.6)
2.7
1.34f
2.7 (1.1– 6.8)
2.4 (1.0 –5.7)
3.7 (1.4 –10.0)
BMI at age 18, family history
of breast cancer, age at
menarche, parity/age at first
birth, and past HT
Estradiol
Age, BMI, age at menarche,
first birth, menopause,
family history of breast
cancer, physical activity,
surgical menopause, alcohol
Bioavailable estradiol
Age, storage time, subcohort,
parity, and oophorectomy
Estradiol, SHBG
Age at menarche, family
history of breast cancer,
parity/age at first birth,
history of total
oophorectomy, history of
breast cancer
Estrone
Age
Estradiol
Years since menopause, height,
weight, parity, family
history of breast cancer
Age at menarche, parity, BMI,
years postmenopausal,
Estradiol
Total estradiol, SHBG-bound
estradiol
Age
e
5.9 (1.6 –21.9)
N/A
Adjusted RR by other variables
N/A
g
Unadjusted effect
ratios (95% CI)
1.1 (0.9 –1.3)
1.9 (1.1–3.3)
2.4 (1.4 – 4.0)
2.1 (0.9 – 4.7)
1.9 (1.1–3.3)
1.1 (0.5–2.3)
3.3 (1.1–10.3)
1.4 (0.7–2.7)
0.8 (0.3–2.4)
1.2 (0.4 –3.5)
Not different
5.7 (1.5–22.2)
5.9 (1.2–29.3)
6.2 (2.0 –19.0)
1f
N/Ad
Effect ratiosa
(95% CI)b
T, Testosterone; N/A, not applicable because comparison was mean difference between cases and controls; RH, relative hazard; HT, hormone therapy; Not different, adjusting
for these variables had no effect; BMI, body mass index.
a
Effect ratios were presented as RR for Garland, Berrino, Dorgan, and Hankinson studies; as OR for Thomas and Zeleniuch-Jacquotte studies; and as RH for Cauley study.
b
Effect ratios of total testosterone, except for Berrino and Cauley studies, in which effect ratios were for free testosterone.
c
Mean age of all participants.
d
No statistically significant difference of testosterone levels between cases and controls by mean comparison.
e
Data not available.
f
95% CI was not available.
g
Reanalysis after incident case was defined as at least 2-yr interval after blood taken resulted in RR ⫽ 1.3 (0.5–3.4).
h
Median age at diagnosis was 66.1 yr.
Hankinson,
1998 (98)
All
2.9
3.5
All
All
9.0
2.3
Mean or
median
time to
diagnosis
(yr)
Excluded new cases
diagnosed within
6 months of
recruitment
All
Definition of study
case
Thomas, 1997
(81)
Garland, 1992
(82)
Berrino, 1996
(95)
Dorgan, 1996
(96)
Wysowski,
1987 (86)
First author,
year (Ref.)
TABLE 5. Prospective studies of the association between plasma testosterone levels and risk of breast cancer in postmenopausal women
Somboonporn and Davis • Testosterone Effects on the Breast
Endocrine Reviews, June 2004, 25(3):374 –388
383
384
Endocrine Reviews, June 2004, 25(3):374 –388
reduced risk for breast cancer in young women in whom the
number of GGC repeat lengths was greater than 17. In addition, they also suggested that AR repeat length (CAG or
GGC) may be partly responsible for the increased risk for
early-onset breast cancer in women who use oral contraceptives, although these findings need replication in other populations (116).
VI. Aromatization and Breast Cancer Development
Estrogen biosynthesis in postmenopausal women is primarily the result of aromatization of circulating C19 steroids
in extragonadal sites (1). The activity of the aromatase enzyme increases with age in fat tissue (117), and with increasing age there is greater fat tissue in the breast. Thus, hypothetically higher androgen levels in women provide
increased substrate for estrogen biosynthesis within the
breast. Despite this, it is also evident that estrogen itself can
block its own bioformation in both human breast cancer cells
and animal studies. Nakamura et al. (118) have shown that
ovariectomy increases and estradiol treatment decreases aromatase activity in baboon mammary tissue. Subsequently, an
inverse correlation between tumor aromatase activity and
estrogen content has been reported in nude mice bearing
xenografts of MCF-7 cells transfected with the aromatase
gene (119). Moreover, an in vitro study in which MCF-7 cells
were cultured long term in an estrogen-deprived medium
and called by the acronym LTED cells found that long-term
estrogen deprivation enhanced aromatase activity by 3- to
4-fold when compared with the wild-type MCF-7 cells (119).
Reexposure of LTED cells to estrogen resulted in reducing
aromatase activity to the levels of the wild-type MCF-7 cells
(119). Consistent with these findings, intratumoral aromatase activity was higher in women with lower circulating
estrogen (120, 121), and relative low activity was found in the
patients taking hormone therapy (119, 122). These data suggest that after menopause, when circulating estrogen levels
are low, an increase in aromatase levels in the breast may
maintain tissue concentrations of estrogen. Thus, aromatase
may control the local production of estrogen through an
autocrine loop. During the process of transformation to malignancy, locally produced estrogen may stimulate the proliferation of tumor cells and vascular endothelial growth
factor production. These effects are also likely to enhance
tumor progression, development of angiogenesis, and, ultimately, metastasis of cancer. Therefore, it is more appropriate to use testosterone replacement only in women who have
adequate estrogen replacement.
VII. Should Androgens Be Included in
Postmenopausal Hormone Therapy Regimens?
Whether there is a role for the use of androgens in the
management of postmenopausal women remains controversial. Clinical studies of supraphysiological testosterone
therapy have shown improvements in sexual parameters in
postmenopausal women (123–125). More recent studies employing more physiological doses have shown benefits in
several parameters of sexual function and in mood (126 –128).
Somboonporn and Davis • Testosterone Effects on the Breast
At present, a variety of testosterone-containing preparations
are being used in clinical practice or in investigational research protocols for the treatment of sexual problems in
women. Although the findings of this review indicate favorable effects of nonaromatizable androgens in the breast,
in contrast to testosterone, there are few data from large
well-designed randomized controlled trials to support the
use of methyltestosterone in the management of sexual dysfunction in women (129). Also, data pertaining to the use of
DHT in women are completely lacking. It is clear that
whether or not the effects of testosterone on sexual function
and mood in women are, in part, dependent on aromatization within the brain needs to be elucidated.
Combined oral estrogen-progestin postmenopausal therapy is associated with an increase in breast cancer risk (6, 7,
10). Whether this is an effect of oral estrogen or the inclusion
of progestin is not known. However, the in vitro and in vivo
data we have summarized indicate that in an estrogenreplete environment androgens oppose the unfavorable effects of estrogen in breast tissue. Consistent with this concept,
tibolone, a synthetic steroid with estrogenic, progestogenic,
and androgenic properties does not appear to have any adverse effects on breast tissue in vitro (130). In contrast, a large
cohort study reported a RR of breast cancer of 1.45 (95% CI,
1.25–1.68) for users vs. nonusers of tibolone (131). However,
these data should be viewed cautiously until reevaluated in
a randomized controlled trial because of the inherent bias in
the study population and the tendency for clinicians to prescribe tibolone rather than standard hormone therapy to
women believed to be at increased breast cancer risk (132,
133). The Women’s Health Initiative study reported that for
the 12,304 women in the study who had never previously
been treated with hormone therapy there was no increase in
breast cancer risk for a mean duration of 5.2 yr of estrogenprogestin therapy (hazard ratio, 1.06; 95% CI, 0.81–1.28) (5–
11). The significant increase in breast cancer risk reported in
this study was related to the 4304 prior users (5–11). Thus, a
large study would be required to demonstrate the risks vs.
benefit of adding testosterone therapy to estrogen-progestin
therapy for 5 yr; in addition, to explore any possibility that
there may be a reduction in risk with testosterone, the study
would need to be beyond 5 yr duration (133). An alternative
approach would be to employ surrogate endpoints such as
mammographic density and indices of breast cell proliferation and apoptosis. The latter requires breast needle biopsies
in healthy women, a minor but invasive procedure. Although the findings from such research may be informative,
there is still the danger of misleading results. An analogy is
the substantial evidence that estrogen lowers lipids; yet in
one large randomized controlled trial, estrogen-progestin
therapy was associated with more cardiovascular events (5–
11). Thus, with the available data pertaining to the effects of
testosterone on the breast, the inclusion of testosterone in
hormonal regimens should be limited to women symptomatic of androgen insufficiency despite adequate estrogen replacement. Testosterone therapy for women should involve
regular measurements of circulating levels of free testosterone, and levels should be maintained below the upper limit
of the normal physiological range for young women to avoid
androgen excess (4).
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Somboonporn and Davis • Testosterone Effects on the Breast
Endocrine Reviews, June 2004, 25(3):374 –388
VIII. Conclusion
Breast cancer has complex etiologies; however, endogenous sex steroids clearly have a role in the progression of this
disease. In vitro and in vivo studies indicate that both testosterone and DHT have a predominantly inhibitory influence
on the mitogenic and cancer-promoting effects of estrogen in
breast cells and promote apoptosis via the AR. There are,
however, variations in these effects according to the type of
breast cancer cell line studied, the androgen administered,
and the dose used. These differences appear to be a consequence of differing levels of coactivator and corepressor proteins that influence AR actions in different cell types.
Unfortunately, most clinical studies have used total testosterone as a measure of androgen exposure, and these
generally have shown that higher total testosterone levels are
associated with increased breast cancer risk. However, these
findings may reflect higher SHBG levels due to higher endogenous estrogen. There are few data pertaining to the
relationship between free testosterone levels and breast cancer risk in humans using reliable assay methodology. Although studies in both premenopausal and postmenopausal
women are inconclusive, there is no evidence that hyperandrogenism in women with PCOS is associated with increased
breast cancer risk. Data for the use of exogenous testosterone
and breast cancer risk are limited. The strongest supporting
data for exogenous testosterone therapy come from primate
studies. Based on such simulations, inclusion of testosterone
in postmenopausal estrogen-progestin regimens has the potential to ameliorate the stimulating effects of combined
estrogen-progestin on the breast. Research addressing this is
warranted; however, the number of women that would be
required for an adequately powered randomized controlled
trial renders such a study unlikely. Unless more specific data
become available, the use of testosterone should be limited
to women symptomatic of androgen insufficiency despite
adequate estrogen replacement.
5.
6.
7.
8.
9.
10.
11.
12.
13.
14.
15.
16.
17.
Acknowledgments
Address all correspondence and requests for reprints to: Professor
Susan R. Davis, The Jean Hailes Foundation NHMRC Centre of Clinical
Research Excellence, 173 Carinish Road, Clayton, Victoria 3168, Australia. E-mail: [email protected]
18.
19.
20.
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